What Is the World Made Of?

Introduction

Is light a wave or a particle?

Light (and other forms of electromagnetic radiation) has the properties of both a particle and a wave. As a wave it is described by its wavelength, frequency, amplitude, and polarization. It has the ability to diffract and interfere. As a particle it has energy, momentum, and angular momentum. It has the ability to be emitted and absorbed, and to scatter off other particles, transferring energy and momentum. In some experiments it acts like a wave in part of the experiment and a particle in other parts.

For example, if you put a beam of light through a pair of narrow, closely spaced slits, the so-called Young two-slit experiment, you get an interference pattern with alternating stripes of light, where the light through the two slits constructively interferes, and darkness where the interference is destructive. If you now greatly reduce the intensity of the light and use a detector that can detect individual photons you will have regions where a large number of photons arrive and others where none arrive. The regions are exactly where the dark and light stripes were.

If the intensity is so low that there is only one photon in the apparatus at a time how can the dark and light regions be understood?

Particles can’t split, with half going through one slit, half through the other so the two halves interfere. If you try to modify the experiment so you can tell through which slit the photon came you destroy the interference pattern.

Physicists have grappled with this mystery for decades. There are no easy explanations. Perhaps the fault lies with our language and thinking. We simply do not have the correct words or mental concepts to describe and understand how light works.